icon: Inkwell's mark is a black inkpot and quill on the brand yellow

Step 5 of milestone 481. Replaces the linked-notes constellation, which had been
stale since note links were dropped (alembic 0024). The colour scheme stays.

packaging/icons.py draws the mark once and renders every variant from it: the
rounded tile (web, desktop), the maskable full-bleed web icon, and the Android
adaptive foreground. The detail (shaft, vane splits, glint) is cut out of the ink
with a mask rather than painted on in yellow, because the Android foreground is
now transparent and its alpha is also the themed-icon silhouette. The old
foreground was the opaque maskable tile, which a themed icon would have drawn as
a solid square.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/public/icon-maskable.svg

@@ -1,24 +1,30 @@
 <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512" role="img" aria-label="Inkwell">
   <!-- Maskable variant: full-bleed background so platform masks (circle, squircle,
-       rounded-square) never clip the glyph. The graph sits inside the central 80%
-       safe zone. Same mark as icon.svg. -->
+       rounded-square) never clip the mark, which sits inside the central 80% safe
+       zone. Same mark as icon.svg. -->
   <rect width="512" height="512" fill="#F5C518"/>
-  <g stroke="#18181b" stroke-linecap="round" fill="none">
-    <g stroke-width="14" opacity="0.26">
-      <path d="M256 150 L166 336"/>
-      <path d="M166 336 L346 336"/>
-      <path d="M346 336 L256 150"/>
-    </g>
-    <g stroke-width="22">
-      <path d="M256 268 L256 150"/>
-      <path d="M256 268 L166 336"/>
-      <path d="M256 268 L346 336"/>
+  <g transform="translate(256 256) scale(0.8) translate(-262 -226)">
+  <defs>
+    <mask id="inkwell-cuts" maskUnits="userSpaceOnUse" x="0" y="0" width="512" height="512">
+      <rect width="512" height="512" fill="#fff"/>
+      <g stroke="#000" fill="none">
+      <g transform="translate(256 232) rotate(34)">
+        <path d="M1 -52 L3 -232" stroke-width="6" stroke-linecap="round"/>
+        <path d="M62 -128 L30 -108 M64 -176 L32 -152" stroke-width="8"/>
+      </g>
+      <path d="M170 344 C 172 318 188 300 214 292" stroke-width="10" stroke-linecap="round"/>
+      </g>
+    </mask>
+  </defs>
+  <g mask="url(#inkwell-cuts)">
+    <g transform="translate(256 232) rotate(34)">
+      <path d="M0 6 L0 -46" stroke="#18181b" stroke-width="12" stroke-linecap="round"/>
+      <path d="M0 -34 C 46 -78 58 -176 4 -252 C -34 -196 -40 -104 0 -34 Z" fill="#18181b"/>
     </g>
+    <path fill="#18181b" d="M222 262 C 166 268 140 300 140 338 L140 374 C140 396 156 410 178 410
+       L334 410 C356 410 372 396 372 374 L372 338 C372 300 346 268 290 262 Z"/>
+    <rect x="214" y="236" width="84" height="34" rx="9" fill="#18181b"/>
+    <rect x="200" y="226" width="112" height="22" rx="11" fill="#18181b"/>
   </g>
-  <g fill="#18181b">
-    <circle cx="256" cy="150" r="34"/>
-    <circle cx="166" cy="336" r="34"/>
-    <circle cx="346" cy="336" r="34"/>
-    <circle cx="256" cy="268" r="26"/>
   </g>
 </svg>